1. Field of the Invention
This invention relates to capacitance type displacement measuring instruments, and more particularly to improvements in a capacitance type displacement measuring instrument wherein a charge in electric capacitance between electrodes due to a relative displacement between two members movable relative to each other results in a change in phase of a detection signal, and the relative displacement between the two members is determined from the change in phase.
2. Description of the Prior Art
Measuring instruments for measuring a length or the like of an article, wherein the movement of articles movable relative to each other, such as the movement of a measuring element to a main body or movement of a slider to a column is measured, are known. For example, capacitance type displacement measuring instruments, in which a frame member holding a main scale is held in one hand and a detector including an index scale is held in the other hand, and a relative displacement value is read by an electrostatic method have been used.
Of these capacitance type displacement measuring instruments (in which a change is electric capacitance between the electrodes due to a relative displacement between two members movable relative to each other is detected on the basis of the change in phase of a detection signal and the relative displacement between two members is determind from the change in phase), there has been proposed one similar to the present invention, in which there are used two sine wave pattern electrodes having the forward end portions formed into complementary sine wave patterns, as disclosed in U.S. Pat. No. 3,068,457 for example. However, according to this U.S. Pat. No. 3,068,457, it is necessary that at least two sets of sine wave pattern electrodes be provided on two members movable relative to each other in the widthwise directions thereof. Thus, four sets of sine wave pattern electrodes must be provided in the embodiment thereof. This has been unsuitable for use in a compact sized displacement measuring instrument.
Furthermore, in Japanese Patent Laid-Open No. 94354/79 (corresponding to U.S. Pat. No. 4,420,754), there has been proposed one in which only one set of plates may be provided on two members movable relative to each other in the widthwise direction thereof. However, in this case, a polysphase oscillator of three phases or more is required, and further, when the digital process is conducted, the circuit arrangement becomes disadvantageously complicated.